Showing papers in "Journal of Guidance Control and Dynamics in 1992"

Nonlinear inversion flight control for a supermaneuverable aircraft

Abstract: Nonlinear dynamic inversion affords the control system designer a straightforward means of deriving control laws for nonlinear systems. The control inputs are used to cancel unwanted terms in the equations of motion using negative feedback of these terms. In this paper, we discuss the use of nonlinear dynamic inversion in the design of a flight control system for a Supermaneuvera ble aircraft. First, the dynamics to be controlled are separated into fast and slow variables. The fast variables are the three angular rates and the slow variables are the angle of attack, sideslip angle, and bank angle. A dynamic inversion control law is designed for the fast variables using the aerodynamic control surfaces and thrust vectoring control as inputs. Next, dynamic inversion is applied to the control of the slow states using commands for the fast states as inputs. The dynamic inversion system was compared with a more conventional, gain-scheduled system and was shown to yield better performance in terms of lateral acceleration, sideslip, and control deflections.

Discrete approximations to optimal trajectories using direct transcription and nonlinear programming

Abstract: A recently developed method for solving optimal trajectory problems uses a piecewise-polynomial representation of the state and control variables, enforces the equations of motion via a collocation procedure, and thus approximates the original calculus-of-variations problem with a nonlinear-programming problem, which is solved numerically. This paper identifies this method as a direct transcription method and proceeds to investigate the relationship between the original optimal-control problem and the nonlinear-programming problem. The discretized adjoint equation of the collocation method is found to have deficient accuracy, and an alternate scheme which discretizes the equations of motion using an explicit Runge-Kutta parallel-shooting approach is developed. Both methods are applied to finite-thrust spacecraft trajectory problems, including a low-thrust escape spiral, a three-burn rendezvous, and a low-thrust transfer to the moon.

Benchmark problems for robust control design

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Control system optimization using genetic algorithms

Abstract: The use of genetic algorithms as a technique for solving aerospace-related control system optimization problems is explored in this paper. Genetic algorithms are parameter search procedures based on the mechanics of natural genetics. They combine a Darwinian survival-of-the-fittest strategy with a random yet structured information exchange among a population of artificial chromosomes. The genetic algorithm technique is used to design a lateral autopilot and a windshear controller. The results show that a variety of aerospace control system optimization problems can be addressed using genetic algorithms with no special problem-dependent modifications. Suggestions for other uses related to aerospace control system optimization are presented.

Optimal vibration control by the use of piezoceramic sensors and actuators

Abstract: In this paper, a discrete degrees of freedom model has been formulated for a structural dynamic system consisting of a linear elastic structure, bonded piezoceramic sensors and actuators, and a feedback signal conditioning system. In addition, an optimal control procedure based on the minimization of a quadratic performance index of state and control vectors has been developed that uses output feedback methods. Finally, the application of the model and the control technique has been demonstrated through the example of a linear elastic beam with piezoceramic sensors and actuators occupying discrete subdomains of the beam upper and lower surfaces. A model for the linear elastic beam has been obtained by using test results and a structural dynamic system identification method based on an equation error approach. Results for various weights in the performance index are included, and implications for future applications are discussed. N the past few years, there has been considerable research activity in the field of active and passive control of vibra- tions of flexible structures. One of the methods of active con- trol of vibrations, termed "electronic damping" in some of the early literature,1'6 involves the placement of piezoceramic devices on a structure to sense and control dynamic strains induced by structural vibrations. The deformation of a sens- ing transducer results in an electrical current that is condi- tioned by operations such as amplification and shifting of the phase of the signal. The conditioned signal is then applied to another piezoceramic, electrostrictive, or magnetostrictive device placed at a selected location on the structure. This trans- ducer acts as an actuator and transmits mechanical energy to the structure. Depending on the applied voltage, electrome- chanical coupling of the forcing transducer to the structure, and the location of the transducers, a degree of vibration control of flexible structures can be achieved. To date, applica- tions of the aforementioned scheme have primarily been in the area of large space structures, such as in the work of Crawley and Deluis,7 but the scheme is applicable to any structure with lightweight components. This type of active control offers unique features that are not usually employed for control of structural vibrations. The dynamics of direct contact type sensors and actuators permit a wide frequency range of control. A measure of tunability is provided for the control of structural systems that age or grow. Finally, this method adds little mass to the controlled flexible structure so that the existing plant model does not need to be modified to account for the mass of the transducers. To utilize the advantages of piezoceramic transducers, it is necessary to select appropriate positions of the transducers and to select the sensor signals that are to be fed back to the actuators. The problem of selecting the locations of the transducers is a complete problem in itself and thus will not be addressed in this paper. There has been some work in

Robust Time-Optimal Control of Uncertain Flexible Spacecraft

Abstract: A new approach for computing time-optimal open-loop control inputs for uncertain flexible spacecraft is developed. In particular, the single-axis, rest-to-rest maneuvering problem of flexible spacecraft in the presence of uncertainty in model parameters is investigated. Robust time-optimal control inputs are obtained by solving a parameter optimization problem subject to robustness constraints. A simple dynamical system with a rigidbody mode and one flexible mode is used to illustrate the concept.

Robust fault detection of jet engine sensor systems using eigenstructure assignment

Abstract: This paper examines a robust fault detection scheme that can be used to detect faulty sensors of jet engines. The fault detection scheme has to be insensitive to disturbances while being highly sensitive to sensor faults (robust). The paper presents a complete description of a robust fault detection approach based on eigenstructure assignment, both in continuousand discrete-time domains. By assigning the left (or right) eigenvectors of the observer orthogonal (or parallel) to the disturbance directions, the robust (disturbance decoupling) fault detection is achieved. The approach has been applied to a realistic jet engine simulation system. The system is a 17th-order system, and a reduced-order model is used to approximate the system. Modeling errors are considered as disturbances acting on the fault detection scheme. A particularly novel feature of the work is the development and use of a new method (new in this context) for estimating disturbance direction. The robust fault detection scheme design uses this estimated direction as that of the direction of unknown inputs (disturbances). Simulation results show that the scheme can detect soft or incipient faults efficiently.

Method for Optimal Actuator and Sensor Placement for Large Flexible Structures

Abstract: A method of finding the optimal sensor and actuator locations for the control of flexible structures is presented. The method is based on the orthogonal projection of structural modes into the intersection subspace of the controllable and observable subspaces corresponding to an actuator/sensor pair. The controllability and observability grammians are then used to weight the projections to reflect the degrees of controllabili ty and observability. This method produces a three-dimensio nal design space wherein sets of optimal actuators and sensors may be selected. A novel parameter is introduced that is potentially useful for studying the problem of the number of actuators and sensors, in addition to their optimal locations. UPPOSE a specific number of actuators and sensors is given and they are placed at specific locations on a flexible structure such that the effectiveness of the chosen actuator and sensor locations could be analyzed. If it turns out that the a priori chosen number and locations for the actuators and sensors are not sufficiently effective, the question naturally arises as to how the locations could be changed to improve the system. Furthermore, it is possible that the a priori number of actuators and/or sensors used is insufficient or redundant. Thus, there is clearly a need for a computationall y feasible technique that is capable of determining an optimal set of locations and the minimal number of actuators and sensors. In general, there will be many more candidate locations (perhaps an order of magnitude more) than the number of actuators and sensors actually available. If the number of actuators and sensors is known a priori, all possible combina- tions could be evaluated, and in principle, the global optimum could then be found. Unfortunately, the number of possible combinations increases factorially, and therefore an exhaus- tive search for a global optimal is usually computationally infeasible, while nonlinear programming based techniques typically produce local minimums. In the past, various definitions of the degree of controllabil- ity and observability have been used in guiding the search for optimal actuator and sensor locations. Among these, the de- gree of controllabili ty defined by scalar measures of recovery regions appears useful for the purpose of actuator and sensor placement.1'3 A second approach4 uses the projection magni- tudes of eigenvectors into the input and output matrices to define gross measures of modal controllability and observabil- ity. However, only little attention is given to the development of a systematic search strategy for actually solving for an optimal set of actuators and sensors, and most attention has been directed toward defining what constitutes most suitable actuator and sensor locations. In this paper, the problem of defining and obtaining the optimal actuator and sensor locations is addressed. A method that is based on the controllability and observability of an actuator/sensor pair is introduced. An outline of the present paper follows. First, the model of actuator and sensor loca- tions for a linear, second-order dynamical system is presented. The basic assumption is that we are given a set of significant modes whose control is desired via feedback. In the next section, controllable, observable, and their intersection sub- spaces are presented, which forms the basis of the method presented in the sequel. The following section presents the main results of this paper. A cost function that is based on the weighted projection of structural modes into the intersection subspace of the controllable and observable subspaces is intro- duced, and a simple interpretation in terms of balanced coor- dinates is given. A novel method for selecting optimal sensor and actuator locations based on the preceding cost function is outlined. The weighted projections of the structural modes can be viewed as a scalar field in three-dimensi onal design space wherein a designer can easily select a set of actuators and sensors based on his or her own criteria without resorting to elaborate nonlinear programming strategies. The method also allows for the comparison of many actuator and sensor candi- date locations since the computational effort depends only on the product of the number of actuator and sensor location candidates rather than combinatorially based search strategies whpse computational effort is in the order of factorials. In the next section, the method of finding optimal locations is ap- plied to an existing laboratory structure to demonstrate the algorithm. Finally, a few concluding remarks are given.

Nonlinear control law with application to high angle-of-attack flight

Abstract: This paper proposes a general structure for aircraft control at whose core are several blocks of dynamic inversion of the controlled system. This control law is then applied to a nonlinear model of the high angle-of-attack research vehicle (HARV). The control law itself uses the complete nonlinear aerodata base and has no restrictions on the manner in which the control inputs enter the dynamic equations. An algorithm that handles surface limiting while ensuring a solution to the moment equation inverse is incorporated into the control law. The performance of the control law is demonstrated via simulation of a selected supermaneuver.

Active damping by a local force feedback with piezoelectric actuators

Abstract: This paper summarizes a research in the field of active damping of space structures. The test facility consists of a truss structure provided with two active elements that can be placed in various locations. Each of the active elements consists of a linear piezoelectric actuator collocated with a force transducer. Each active element is controlled in a decentralized manner, with an integral feedback of the force on the voltage applied to the piezoactuator. This control law is always stable and has been found very effective; the damping ratio of the first mode has been increased from 0.003 (open loop) to 0.09 with one actuator.

Time-optimal slewing of flexible spacecraft

Abstract: The tune optimal slewing problem of flexible spacecraft is considered. The system is discretized by the assumed modes method, and the problem is solved for a linearized model in reduced state space by parameter optimization. Optimality is verified by the maximal principle. The linear solution is further used to obtain time optimal solutions for the non-linear problem.

Unified approach to inertial navigation system error modeling

Abstract: Several inertial navigation system error models have been developed and used in the literature. Most of the models are ad hoc models which were needed to solve certain particular problems and were developed for that purpose only. Consequently, the relationship, correspondence, and equivalence between the various models is not evident. This paper presents a new methodology for developing inertial navigation systems error models which also puts all of the known models in the same framework and shows the equivalence between them. The new methodology is based on several choices the developer has to make which uniquely define the error model. This new approach enables the development of all existing models in a unified way, hence the equivalence and correspondence between them is obvious. Moreover, any new model which is of interest can be developed using the methodology presented in this work. In fact, any new model which will ever be developed for the class of systems considered here will fit into the framework described in this paper.

Ideal Proportional Navigation

Abstract: Proportional navigation has been proved to be a useful guidance technique in several surface-to-air and air-to-air homing systems for interception of airborne targets. Besides the familiar pure, true, and generalized proportional navigation guidance laws, a new guidance scheme, called ideal proportional navigation, with commanded acceleration applied in the direction normal to the relative velocity between interceptor and target, is presented. In this study the closed-form solutions of ideal proportional navigation are completely derived for maneuvering and nonmaneuvering targets, and some important characteristics related to the system performance are introduced. Under this scheme the capture criterion is related to the effective proportional navigation constant only, no matter where the initial condition and target maneuver are. With some more energy consumption, this new guidance scheme has a larger capture area and is much more effective than the other schemes.

Global steering of single gimballed control moment gyroscopes using a directed search

Abstract: A guided depth-first search that manages null motion about torque-producing trajectories calculated with a singularity-robust inverse is proposed as a practical feedforward steering law that can globally avoid (or minimize the impact of) singular states in minimally redundant systems of single gimballed control moment gyroscopes. Cost and heuristic functions are defined to guide the search procedure in improving gimbal trajectories. On- orbit implementation of the steering law is proposed as an extension to momentum management algorithms. A set of simulation examples is presented, illustrating the search performance for a minimally redundant, pyramid- mounted array. Sensitivities o£ feedforward gimbal trajectories are examined in the presence of unmodeled disturbances, and techniques are proposed for avoiding excessive divergence. HE next generations of manned and unmanned space- craft will require enhanced control algorithms in order to efficiently achieve their proposed mission objectives. Reg- ularly coping with uncertainties in the orbital environment, dynamically changing spacecraft configurations (i.e., via docking and buildup), nonlinear actuator properties, and the need to tolerate potential hardware failures will mandate develop- ment of control strategies considerably beyond the available state-of-the-art. Because of the priority placed on minimizing cost, weight, and consumable requirements, future spacecraft will not always be able to rely on highly complex, multiply- redundant actuator systems (as is often now the case), but must employ more flexible and intelligent schemes that effi- ciently exploit all available onboard control capability. This study has examined methods of applying heuristic search techniques to perform adaptable inverse kinematics and ac- tuator management for spacecraft attitude control. In partic- ular, single gimballed control moment gyros (SGCMGs) were chosen as torque actuators. Although many existing al- gorithms may suffice for steering double gimballed CMG (DGCMG) arrays, such as envisioned for the NASA Space Station, no powerful techniques have been developed to man- age minimally redundant arrays of SGCMGs. These devices, however, are ideally suited as momentum-exchange effectors for a wide class of future spacecraft because of their large torque output and momentum capacity. They offer significant cost, power, weight, and reliability advantages over DGCMGs and could provide attitude control for a wide variety of future spacecraft, as torque requirements in many proposed mod- erate-sized vehicles may surpass the capability of available reaction wheels. Because of the complicated nonlinear map- ping between the input (gimbal) space and output (momen- tum) space, however, effective steering laws that reliably avoid problematic singular states have not been developed for min- imally redundant SGCMG systems, discouraging their appli- cation in many situations. To fully exploit the capability of SGCMGs, intelligent steering laws must be developed that address the system nonlinearities and avoid singular states over global trajectories.

Application of sparse nonlinear programming to trajectory optimization

Abstract: The most effective numerical techniques for the solution of trajectory optimization and optimal control problems combine a nonlinear iteration procedure with some type of parametric approximation to the trajectory dynamics. Early methods attempted to parameterize the dynamics using a small number of variables because the iterative search procedures could not successfully solve larger problems. With the development of more robust nonlinear programming algorithms, it is now feasible and desirable to consider formulations of the trajectory optimization problem incorporating a large number of variables and constraints. The purpose of this paper is to address the manner in which a trajectory is parameterized and the design of the nonlinear programming algorithm to effectively deal with this formulation.

Robust controller designs for second-order dynamic systems - A virtual passive approach

Abstract: This paper presents a robust controller design for second-order dynamic systems. The controller is model independent and is a virtual second-order dynamic system. The conditions for actuator and sensor placements are identified for controller designs that guarantee overall closed-loop stability. The dynamic controller can be viewed as a virtual passive damping system that serves to stabilize the actual dynamic system. The control gains are interpreted as virtual mass, spring, and dashpot elements that play the same roles as actual physical elements in stability analysis. Position, velocity, and acceleration feedback are considered. Simple examples are provided to illustrate the controller design. From this illustration the physical meaning of the controller design is apparent.

Planar reorientation maneuvers of space multibody systems using internal controls

Abstract: In this paper a reorientation maneuvering strategy for an interconnection of planar rigid bodies in space is developed. It is assumed that there are no exogeneous torques, and torques generated by joint motors are used as means of control so that the total angular momentum of the multibody system is a constant, assumed to be zero in this paper. The maneuver strategy uses the nonintegrability of the expression for the angular momentum. We demonstrate that large-angle maneuvers can be designed to achieve an arbitrary reorientation of the multibody system with respect to an inertial frame. The theoretical background for carrying out the required maneuvers is briefly summarized. Specifications and computer simulations of a specific reorientation maneuver, and the corresponding control strategies, are described. I. Introduction I N this paper we develop a reorientation strategy for a system of N planar rigid bodies in space that are interconnected by ideal frictionless pin joints in the form of an open kinematic chain. Angular momentum preserving controls, e.g., torques generated by joint motors, are considered. The TV-body system is assumed to have zero initial angular momentum. Our earlier work1'2 demonstrated that reorientation of a planar multibody system with three or more interconnected bodies using only joint torque inputs is an inherently nonlinear control problem that is not amenable to classical methods of nonlinear control. The goal of this study is to indicate how control strategies can be explicitly constructed to achieve the desired absolute reorientation of the TV-body system. There are many physical advantages in using internal controls, e.g., joint torque controls, to carry out the desired multibody reorientation maneuvers. First of all, this control approach does not modify the total angular momentum of the multibody system. In addition, internal controls have obvious advantages in terms of energy conservation. Moreover, they can be implemented using standard electrical servo motors, a simple and reliable control actuator technology. The formal development in this paper is concerned with control of a multibody interconnection in space that has zero angular momentum. Although these results are formulated in a general setting, we have been motivated by several classes of specific problems. Several potential applications of our general results are now described. Manipulators mounted on space vehicles and space robots have been envisioned to carry out construction, maintenance, and repair tasks in an external space environment. These space systems are essentially multibody systems satisfying the assumptions of this paper. To carry out the desired tasks, they must be capable of performing a variety of reorientation maneuvers. Previous research on maneuvering of such space multibody systems has mainly focused on maneuvers that achieve desired orientation of some of the bodies, e.g., an end effector, whereas the orientation of some of the remaining bodies cannot be specified, at least using the methodologies employed.3"8 Using the approach suggested in this paper, maneuvers that achieve any desired reorientation for all of the

Recursive flexible multibody system dynamics using spatial operators

Abstract: This paper uses spatial operators to develop new spatially recursive dynamics algorithms for flexible multibody systems. The operator description of the dynamics is identical to that for rigid multibody systems. Assumed-mode models are used for the deformation of each individual body. The algorithms are based on two spatial operator factorizations of the system mass matrix. The first (Newton-Euler) factorization of the mass matrix leads to recursive algorithms for the inverse dynamics, mass matrix evaluation, and composite-body forward dynamics for the systems. The second (innovations) factorization of the mass matrix, leads to an operator expression for the mass matrix inverse and to a recursive articulated-body forward dynamics algorithm. The primary focus is on serial chains, but extensions to general topologies are also described. A comparison of computational costs shows that the articulated-body, forward dynamics algorithm is much more efficient than the composite-body algorithm for most flexible multibody systems.

Minimizing spacecraft attitude disturbances in space manipulator systems

Abstract: This paper presents a technique called the enhanced disturbance map to plan space manipulator motions that will result in relatively low spacecraft disturbances. This subject is of significant concern because future robotic systems used in space may encounter problems due to the dynamic disturbances imposed on their spacecraft by their manipulator motions. Although a spacecraft's attitude control reaction jets can compensate for these dynamic disturbances, jet fuel is a limited resource and excessive disturbances would limit the life of a system. The enhanced disturbance map can aid in understanding this complex problem and in the development of algorithms to reduce disturbances, including ones that use manipulator redundancy to eliminate the disturbances.

Effect of model error on sensor placement for on-orbit modal identification of large space structures

Abstract: A theory is presented for the effect of errors in large space structure prelaunch finite element models on sensor placement for the on-orbit independent identification of a set of selected target modes. The idea of positive net information is introduced. If the net information matrix remains positive definite as sensor locations are deleted from an initial candidate set, the analytical model provides useful information for the identification of the real target modes. If the net information matrix becomes indefinite, the sensor placement analysis based on the prelaunch analytical model will actually detract from the independent identification of the target modes. A bound is presented that can be easily checked after each iteration during which a sensor was deleted to determine positive definiteness of the net information matrix. Numerical examples are used to illustrate the ideas that are presented.

Spacecraft dynamical distribution of fluid stresses activated by gravity-jitter-induced slosh waves

Abstract: The dynamical behavior of fluids, in particular the effect of surface tension on partially filled rotating fluids (cryogenic liquid helium and helium vapor) in a full-scale Gravity Probe-B Spacecraft propellant Dewar tank imposed by various frequencies of gravity jitters, has been investigated. Fluid stress distribution, caused by the excitation of slosh waves and their associated large-amplitude disturbances on the liquid-vapor interface, exerted on the outer and inner walls of a rotating Dewar container also has been investigated. Results show that fluid stress distributions near the outer and inner walls of the rotating Dewar are closely related to the characteristics of slosh waves excited on the liquid-vapor interface in the rotating Dewar tank. This can provide a useful tool for managing spacecraft dynamic control leading toward the control of spacecraft imbalance caused by the uneven fluid stress distribution due to slosh wave excitations at the interface between liquid and vapor propellants.

Aeroelastic stability of slender, spinning missiles

Abstract: The coupled equations of motion that govern the static and dynamic aeroelastic stability of spinning, flexible missiles are derived. A Lagrangian approach is employed which results in nonlinear coupling between the elastic deflections and the rigid-body motion parameters. Various approximations inherent in reducing the equations to a linear set are indicated. Firstorder aerodynamics are formulated for application to a hypersonic missile. Some numerical examples are given that reveal a destabilizing influence of structural damping at certain roll rates for a particular missile configuration.

Survey of experiments and experimental facilities for control of flexible structures

Abstract: This paper presents a survey of ground experiments primarily conducted in the United States and U.S. facilities dedicated to the study of active control of flexible structures. The facilities are briefly described in terms of capability, configuration, size, and instruments. Topics on the experiments include vibration suppression, slewing control, and system identification. The experiments are listed in tables containing the experiment's name, the responsible organization, a brief description of the test article configuration, and the actuator/sensor devices used in the experiment. Selected experiments will be further discussed to help illustrate the control problems. Some of the test facilities dedicated to ground testing of large space structures are discussed in more detail, to give the reader a better appreciation of ground-testin g work. Several research issues are mentioned, including real-time computer systems, test article suspension, and new actuator/sensor technology development.

H-infinity synthesis using a bilinear pole shifting transform

Abstract: An H-inifinity control law design is presented for a benchmark problem consisting of an undamped pair of spring-coupled masses with a sensor and actuator that are not collocated. This simple mechanical system captures many of the salient features of more complex aircraft and space structure vibration control problems. The H-infinity problem formulation enables the issue of stability robustness in the face of large mass and spring constant variation to be directly addressed. Constraints on closed-loop dominant pole locations and settling time are accommodated via a simple s-plane bilinear transform. The transform parameter can give direct control of closed-loop disturbance settling time and controller open-loop pole-zero locations. A four-state H-infinity minimum phase controller was found to meet the given specifications of each design. Detailed analysis on the tradeoffs of sensor noise vs control energy are also presented.

Adaptive Simulator Motion Software with Supervisory Control

Abstract: Concepts for flight simulator motion-drive algorithms range from the most basic to the relatively complex, with little to guide a choice between these, short of implementing them all and choosing the best. To avoid this lengthy process and to put into practice the experience gained in a previous large-scale evaluation exercise, a flexible motion algorithm has been implemented. It can be run as a simple classical algorithm with few free parameters and can be quickly adjusted to yield good motion performance. The more sophisticated adaptive features of the algorithm can then be brought in gradually to improve performance. Various forms of cost functions and adaptive features were formulated and subsequently evaluated on a synergistic 6 degrees-of-freedom motion-base simulator. It was found that a fourth-order cost function coupled with adaptive gain filters yielded the most favorable results. Finally, a supervisory code to ease motion adjustment and to provide a safe interactive interface with the designer is described. This facility permits reduced turnaround time during motion tuning and allows the pilot to experience different motions back-to-back for easier comparison. The supervisory control also allows automatic motion adjustment to different flight conditions. Nomenclature§ aAA

Predictor-corrector guidance algorithm for use in high-energy aerobraking system studies

Abstract: A three-degree-of-freedom predictor-corrector guidance algorithm has been developed specifically for use in high-energy aerobraking performance evaluations The present study reports on both the development and capabilities of this guidance algorithm to the design of manned Mars aero-braking vehicles Atmospheric simulations are performed to demonstrate the applicability of this algorithm and to evaluate the effect of atmospheric uncertainties upon the mission requirements The off-nominal conditions simulated result from atmospheric density and aerodynamic characteristic mispredictions The guidance algorithm is also used to provide relief from the high deceleration levels typically encountered in a high-energy aerobraking mission profile Through this analysis, bank-angle modulation is shown to be an effective means of providing deceleration relief Furthermore, the capability of the guidance algorithm to manage off-nominal vehicle aerodynamic and atmospheric density variations is demonstrated

Dynamics and Control of Maneuverable Towed Flight Vehicles

Abstract: This paper considers the problems of dynamically modeling and automatically controlling the motion of a small maneuverable flight vehicle that is being towed by a much larger one. Mathematical models of the components of the system, including the towing aircraft, tow cable reel mechanism, tow cable, and vehicle aerodynamics, are described. The development of an autopilot for stability augmentation and maneuvering of the towed vehicle is discussed. An overview of the implementation of the models in a digital computer simulation program is provided, and some typical results obtained using the program are presented. A comparison of theoretical and experimental results is also made. ROM the early days of powered flight, there has been an interest in towing objects from aircraft. Objects that have been towed include antennas, banners, gliders, and targets. Because of the interesting, and sometimes troublesome, characteristics of the motions of some towed objects and the cables, or towlines, the dynamics of the cables and the towed objects have been studied by several investigators at various times during the last 60 years.1'10 The problem of describing the motion of a towed cable/ object system is far from trivial. The equations of motion are, in general, coupled, ordinary, and partial differential equations (i.e., hybrid equations). Various approaches have been taken in studying such systems. Glauert1 and Phillips2 did some pioneering work on the stability of the motion of bodies towed from aircraft. Phillips3 also analyzed the stability of the motion of a cable used to tow a nonlifting body. He used a continuum model for the cable and studied the effects of towing speed on the stability of lateral oscillations of the cable and concluded that the oscillations would be damped out if the speed of flight were less than the speed of propagation of vibrational waves along the cable.

Robust H-infinity control synthesis method and its application to benchmark problems

Abstract: This paper presents a robust H-infinity control synthesis method for structured parameter uncertainty. The robust H-infinity control design methodology is also incorporated with the so-called internal model principle for persistent-disturbance rejection. A noncollocated control problem of flexible space structures subject to parameter variations is used to illustrate the design methodology. It is shown that the proposed design method invariably makes use of nonminimum-phase compensation and that it achieves the desired asymptotic disturbance rejection by having a disturbance rejection 'dipole'.